Scope environment readiness config to thread attachments (#38067)

## Why

Threads that share an executor environment can require different capability roots and login-shell policies. Treating readiness configuration as executor-wide can expose one thread's settings to another.

## What changed

- Add `CodexThread::environment_ready` and `EnvironmentConfig` so hosts can install capability roots and login-shell policy for an exact thread, environment, and working-directory attachment.
- Validate attachment readiness updates, merge ready attachment roots with persisted thread roots, and refresh MCP capabilities when those roots change.
- Preserve installed environment policy across thread settings updates and child-thread inheritance while resetting it when the selected working directory changes.

## Testing

- Cover attachment config inheritance and reset behavior.
- Verify that threads sharing an executor retain independent model-visible capability roots and login-shell tool schemas across updates.

GitOrigin-RevId: 60e7a296b3e41b835b0413aa2837451e8a55c230

codex-rs/core/src/session/mcp.rs

@@ -442,11 +442,13 @@ impl Session {
             .selected_capability_roots
             .iter()
             .cloned()
-            .chain(
-                environments
-                    .turn_environments()
-                    .flat_map(|environment| environment.environment.selected_capability_roots()),
-            )
+            .chain(environments.turn_environments().flat_map(|environment| {
+                environment
+                    .config
+                    .selected_capability_roots
+                    .clone()
+                    .unwrap_or_else(|| environment.environment.selected_capability_roots())
+            }))
             .enumerate()
         {
             if let Some(kept_location) = root_locations_by_id.get(&root.id) {